Method, use and device concerning cladding tubes for nuclear fuel and a fuel assembly for a nuclear pressure water reactor

ABSTRACT

A method of producing a cladding tube for nuclear fuel for a nuclear pressure water reactor includes forming a tube which at least principally consists of a cylindrical tube component of a zirconium-based alloy, where the alloying element, except for zirconium, which has the highest content in the alloy is niobium, wherein the niobium content in weight percent is between about 0.5 and about 2.4 and wherein no alloying element, except for zirconium and niobium, in the alloy, has a content which exceeds about 0.2 weight percent. The cladding tube is then annealed such that the tube component is partly but not completely recrystallized. The degree of recrystallization in the tube component is higher than about 40% and lower than about 95%. A fuel assembly for a nuclear pressure water reactor also has a plurality of such cladding tubes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/533,467, filed on Apr. 29, 2005, the contents of which areincorporated herein by reference in their entirety. This application isalso entitled to the benefit of and incorporates by reference essentialsubject matter disclosed in International Application No.PCT/SE2003/001685 filed on Oct. 30, 2003 and Swedish Patent ApplicationNo. 0203198.7 filed on Oct. 30, 2002.

FIELD OF THE INVENTION

The present invention concerns cladding tubes for nuclear fuel for anuclear pressure water reactor. More precisely, the invention concernssuch cladding tubes which are formed of a Zr-based alloy which containsNb. The invention concerns, inter alia, a method. According to themethod, a tube is formed which at least principally consists of acylindrical tube component of a Zr-based alloy, where the alloyingelement, except for Zr, which has the highest content in the alloy isNb, wherein the Nb content in weight percent is between 0.5 and 2.4.

The invention also concerns a cladding tube as such, a use of a claddingtube and a fuel assembly for a nuclear pressure water reactor comprisingsuch a cladding tube.

BACKGROUND OF THE INVENTION

Methods of the kind which is described above are known. With suchmethods, cladding tubes of Zr-based alloys which contain Nb are thusproduced. For example, U.S. Pat. No. 5,648,995 describes such a methodand a cladding tube of this kind.

When a cladding tube is used in a nuclear reactor, it contains nuclearfuel, usually in the form of pellets comprising enriched uranium,usually in the form of UO₂. The cladding tube with its content thusconstitutes a fuel rod. Because of the very particular environment inwhich cladding tubes are used, different requirements must be fulfilled.

There are mainly two kinds of modern light water reactors: boiling waterreactors (BWR) and pressure water reactors (PWR). In these kinds ofreactors different conditions prevail which call for differentrequirements on the parts which form part of the reactors. In a PWR, thefuel rods are cooled mainly by water that is in a liquid phase underhigh pressure. In a BWR, the pressure is lower and the water which coolsthe fuel rods is evaporated such that the fuel rods are surrounded bothby water in a liquid phase and in a steam phase. Furthermore, the fuelassemblies have different construction in a BWR and a PWR. In one kindof BWR, the fuel rods in a fuel assembly extend all the way between atop plate and a bottom plate which keep the fuel assembly together. In aPWR, on the other hand, the fuel rods are usually held in position withthe help of spacers and do not reach all the way to the top plate andthe bottom plate.

A fuel rod which is used in a nuclear reactor is exposed to hightemperatures and pressures. Over time thereby creep phenomena occur.Such a creep should as far as possible be avoided since it can havenegative effects. For example, a creep of the fuel rods may have as aconsequence that they will press against the fuel pellets which arelocated therein. The neutron radiation to which a fuel rod is exposedwhen it is used may also have as a consequence that the fuel rod tendsto grow with time. Also such a growth caused by neutron radiation mayhave undesired effects. It should therefore be avoided that the claddingtube grows to a larger extent. Modern fuel rods which are produced insuitable zirconium alloys and which undergo special heat treatmentsduring the production often have a relatively low tendency to grow whenthey are exposed to neutron radiation. The tendency to grow may bereduced, inter alia in that the cladding tube during the productionundergoes a final recrystallization anneal.

Through a suitable choice of the material for the cladding tube and asuitable method of production, the cladding tube can obtain suitableproperties concerning for example hardness and ductility.

In the environment where the cladding tubes are used they may be subjectto different corrosive attacks. These attacks may come from the outsideor from the inside. The attacks from the inside often have their basisin an influence from the nuclear fuel material that is located there,so-called pellet-cladding interaction (PCI). If a crack is formedthrough the cladding tube (a so-called primary damage), water maypenetrate in through the crack and spread along the inside of the tube.This may lead to new corrosive attacks from the inside of the tube,so-called secondary damages. A cladding tube of zirconium orzirconium-based alloys may also react with hydrogen such that hydridesare formed in the cladding tube. These hydrides may be formed from theinside of the tube, particularly if a crack has been formed such thatwater has penetrated into the tube. These hydrides make the tube morefragile and the probability for the formation of cracks increases.Particularly hydrides that extend in a radial direction through the tubeconstitute an increased risk for crack formation. Such radial hydridesmay therefore speed up possible secondary damages and crack formations.

The complicated chemical, mechanical and metallurgical conditions thatare the case in a nuclear reactor have lead to the fact that a verylarge number of suggestions have been proposed for the selection ofmaterials and for the methods of production of cladding tubes. Evensmall changes in the composition of alloys or production parameters mayhave a large importance for the properties of the cladding tube.

SUMMARY OF THE INVENTION

Cladding tubes produced of a Zr-based alloy which contains Nb hasappeared to have good properties in many respects. By suitable alloyingcontents (for example such as described in the above mentioned U.S. Pat.No. 5,648,995) and by a suitable choice of parameters of production, acladding tube can be obtained which has good chemical, mechanical andmetallurgical properties. It has however become clear that also fortubes of this kind there is a risk of damages.

An object of the present invention is therefore to achieve a method ofproducing a cladding tube, of a Zr-based alloy which includes between0.5 weight percent and 2.4 weight percent Nb and which has an improvedresistance against damages than prior cladding tubes of this kind ofalloys.

These objects are achieved by a method characterized in that after thatthe cladding tube has been formed according to the above and afterpossible rolling steps with heat treatments between them, the claddingtube is finally annealed at a temperature and during a time such thatsaid tube component is partly recrystallized but not completelyrecrystallized. The tube component is thus partially recrystallized(pRXA).

A cladding tube produced according to this method has appeared to have agood resistance against damages caused by PCI at the same time as therisk for the formation of radial hydrides is low. Thereby, the risk forcracks is reduced. The cladding tube has at the same time also a highductility, a low creep rate and a low tendency to growth caused byneutron radiation. Further objects and advantages of the invention willbecome clear from the following.

Since the tube component is pRXA (and not completely recrystallized), ithas become clear that hydrides which are formed tend to extend in mainlya tangential direction while the risk for radial hydrides is low.Thereby, an improved resistance against crack formation is obtained. Thereason why radial hydrides are avoided is probably that certain tensionswhich originate from the production of the tube are maintained since thetube component is not completely recrystallized. These tensions have aconsequence that the tendency for radial hydrides is reduced.

It can be noted that previously known cladding tubes of this kind ofalloys have undergone a final anneal such that the cladding tube hasbecome completely recrystallized (see for example the above mentionedU.S. Pat. No. 5,648,995). Such an RXA is advantageous in certainrespects (for acting against creep and growth caused by neutronradiation and for achieving resistance against PCI damages). However,the inventors of the present invention have found that these advantagesto a large extent can be obtained also if the cladding tube is onlyfinally annealed for achieving pRXA. It has thus thereby become clearthat an improved resistance against damages may be obtained through thisfinal anneal.

It should be noted that the final anneal is normally the last heattreatment step in the method of production. Possibly, a certain aftertreatment of the cladding tube may be carried out, but such an aftertreatment should be such that the structure which is obtained throughthe final anneal is not essentially destroyed.

It should also be noted that according to a preferred embodiment, thecladding tube consists only of said tube component. There are thus nofurther layers. The composition of the outer surface and the innersurface of the tube may however differ from the composition in the innerof the tube, for example due to the substances that the tube has comeinto contact with. The tube may for example be oxidised through the factthat it has been kept in an environment of air. According to analternative embodiment, it is however feasible that the tube comprisesone or more further protective layers on its inside or its outside. Inthis case, the tube thus consists of several components. It is howeveralways the case that said tube component constitutes the main componentof the tube, for example that this tube component constitutes more than60% of the thickness of the tube. As has been pointed out above, it ishowever preferred that the whole thickness of the tube is made up ofsaid tube component.

As used herein % or ppm are used in connection with contents ofdifferent substances, it is, if nothing else is said, referred to weightpercent of the respective substances.

According to a preferred manner of carrying out the method according tothe invention, the final anneal is carried out such that the degree ofrecrystallization in the tube component is higher than 5% and lower than95%, preferably higher than 40%, for example between 60% and 90%. It hasbecome clear that such degrees of recrystallization are particularlysuitable for achieving the described advantages.

The temperature and the time that are needed in order to achieve such adegree of recrystallization depend on the contents of the alloyingelements. The temperature for the final anneal is preferably lower than550° C., for example between 400° C. and 540° C., and often mostpreferred between 450° C. and 500° C. The final anneal may suitably becarried out during 1 h to 6 h, preferably during 1 to 3 hours.

According to a preferred manner, the method comprises, before said finalanneal, the following steps:

-   -   a bar of said Zr-based alloy is formed;    -   this bar is heated to between 900° C. and 1300° C. and is        thereafter quenched, preferably in water;    -   a billet is extruded from the bar after heating to between        500° C. and 900° C.;    -   the billet is cold rolled to a tube in at least two steps, with        heat treatments between them at between 550° C. and 650° C.

Such a method of production is suitable in order to obtain favourableproperties of the cladding tube. It should be noted that the method ofproduction of course may comprise further steps (for example furtherheat treatments or cold rolls) in addition to those mentioned above.

According to a preferred manner, the Nb-content in said alloy is between0.8 weight percent and 1.2 weight percent. Preferably, no alloyingelement, except for Zr and Nb, in said alloy has a content which exceeds0.3 weight percent, and preferably not above 0.2 weight percent.

The alloy may suitably contain between 800 ppm and 1700 ppm O. Such aselection of the content of O leads to the fact that the cladding tubehas good creep properties.

According to an advantageous embodiment, the alloy contains between 50ppm and 600 ppm Fe. By keeping the content of Fe low, the creepproperties are further improved. The Fe-content may for example be lowerthan 250 ppm. It should be noted that these low Fe-contents are onlypreferred embodiments of the invention. According to another embodiment,also a higher Fe-content may be permitted. The alloy may also contain acertain amount of S, for example between 20 ppm 5 and 600 ppm S, orbetween 100 ppm 5 and 600 ppm S. Such an amount of S can improve thecorrosion resistance of the alloy and the creep properties.

According to a preferred embodiment, said alloy contains, in addition toZr, 0.8 weight percent to 1.2 weight percent Nb, 50 ppm to 600 ppm Fe,800 ppm to 1700 ppm O, less than 250 ppm C, less than 150 ppm Si, lessthan 1000 ppm S and in addition to that only impurities of a contentwhich does not exceed that which is normally accepted in Zr or Zr alloysfor applications in nuclear reactors.

Examples of what is considered as acceptable impurities in this contextare mentioned for example in the patent document EP 0 674 800 B1, column5.

The present invention also resides in a cladding tube produced accordingto the method described in any of the preceding embodiments is used in afuel assembly for a nuclear pressure water reactor. Thereby the abovedescribed advantages with such a cladding tube are achieved.

The invention also concerns a cladding tube as such, suitable to containnuclear fuel for a nuclear pressure water reactor, which cladding tubeat least principally consists of a cylindrical tube component of aZr-based alloy, where the alloying element which, except for Zr, has thehighest content in the alloy is Nb, wherein the Nb content in weightpercent is between 0.5 and 2.4, wherein said tube component has beenfinally annealed such that it has a structure such that it is partlyrecrystallized but not completely recrystallized. The degree ofrecrystallization in the tube component is higher than 5% and lower than95%, preferably higher than 40%, for example between 60% and 90%.

Such a cladding tube can be produced according to the above-describedmethod. Advantageous embodiments, for example concerning includedalloying elements and alloying contents, are clear from the examplesabove in connection with the method according to the invention. Withthese embodiments of the cladding tube, the above-described advantagesare achieved.

Finally, the invention also concerns a fuel assembly for a nuclearpressure water reactor. The fuel assembly comprises a plurality ofcladding tubes according to the invention filled with nuclear fuelsuitable for such cladding tubes for a nuclear pressure water reactor.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a fuel assembly for a nuclear pressure waterreactor.

FIG. 2 shows schematically a cross-section through a cladding tubeaccording to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows schematically a fuel assembly for a PWR. The fuel assemblycomprises a top plate 4 and a bottom plate 5. Between the top plate 4and the bottom plate 5 a plurality of guide tubes 3 for control rodsextend. Furthermore, the fuel assembly comprises a plurality of claddingtubes 1. These cladding tubes 1 thus contain a nuclear fuel material andare thereby called fuel rods. In this kind of fuel assembly for PWR, thefuel rods do not reach all the way to the top plate 4 and to the bottomplate 5. The fuel rods are kept in position in the fuel assembly withthe help of spacers 2.

FIG. 2 shows schematically a cross-section through a cladding tubeaccording to the invention. The cross-section shows the cladding tubestrongly enlarged. In reality, the cladding tube is of a dimension andof a length which are suitable for use in a PWR. The cladding tubecomprises a cylindrical tube component 1. In the shown case, thecylindrical tube component 1 constitutes the whole cladding tube. Thisis the preferred embodiment. As has been mentioned above, it is howeverpossible that this tube component 1 has one or more protective layers onits inside or outside. The tube component 1 consists of a Zr-basedalloy. This means that the tube component to the largest extent, alwaysmore than 95%, consists of Zr. According to an embodiment, the tubecomponent 1 contains the following alloying elements: 1% Nb, 1200 ppm O,200 ppm Fe, less than 200 ppm C, less than 150 ppm Si, less than 1000ppm S and in addition to that only impurities of a content which doesnot exceed that which is normally accepted in Zr or Zr alloys forapplications in nuclear reactors. The cladding tube has been finallyannealed such that the tube component 1 has a structure such that it ispartly recrystallized but not completely recrystallized. The degree ofrecrystallization may for example be about 85%.

The invention also concerns a method of producing a cladding tube fornuclear fuel for a nuclear pressure water reactor. The method accordingto the invention may be carried out in the following manner.

A bar of for example the above-mentioned alloy is formed. This bar isheated to between 900° C. and 1300° C. and is thereafter quenched,preferably in water. A billet is extruded from the bar after heating tobetween 500° C. and 900° C. The billet is cold rolled to a tube in atleast two steps (for example in three steps), with heat treatmentsbetween them at between 550° C. and 650° C. The tube is final annealedat a temperature and during a time such that the tube component ispartly recrystallized but not completely recrystallized. The finalanneal may for example be carried out at a temperature of about 490° C.during about two hours. The final anneal is carried out such that asuitable degree of recrystallization is obtained in the tube. Thisdegree of recrystallization ought to be higher than 5% and lower than95%. A degree of recrystallization of above 40%, for example between 60%and 90% can be suitable, for example a degree of recrystallization ofabout 85%.

A cladding tube produced according to the method may suitably be used ina fuel assembly in a nuclear PWR.

When a fuel assembly of for example the above-described kind is suppliedwith a plurality of cladding tubes according to the invention, a fuelassembly according to the invention is thus obtained.

The invention is not limited to the above given examples but may bevaried within the scope of the following claims.

1. A method of producing a cladding tube for nuclear fuel for a nuclear pressure water reactor, which method comprises the following steps: formation of a tube which at least principally consists of a cylindrical tube component of a Zr-based alloy, where the alloying element, except for Zr, which has the highest content in the alloy is Nb, wherein the Nb content in weight percent is between about 0.8 and about 1.2 and wherein no alloying element, except for Zr and Nb, in said alloy, has a content which exceeds about 0.2 weight percent, and carrying out a final anneal of the cladding tube at a temperature and during a time such that said tube component is partly recrystallized but not completely recrystallized, and wherein said final anneal is carried out such that the degree of recrystallization in said tube component is higher than about 40% and lower than about 95%.
 2. A method according to claim 1, wherein between said formation step and said final annealing step, said method includes the steps of rolling and heat treating said cladding tube.
 3. A method according to claim 1, wherein the final anneal is carried out at a temperature which is lower than 550° C.
 4. A method according to claim 1, wherein the final anneal is carried out at a temperature which is between about 400° C. and about 540° C.
 5. A method according to claim 4, wherein the final anneal is carried out at a temperature which is between about 450° C. and about 500° C.
 6. A method according to claim 1, wherein the final anneal is carried out such that the degree of recrystallization in said tube component is higher than about 60% and lower than about 90%.
 7. A method according to claim 1, wherein the final anneal is carried out during about 1 h to about 6 h.
 8. A method according to claim 1, wherein before said final anneal, the method comprises the following steps: forming a bar of said Zr-based alloy; heating the bar to between about 900° C. and about 1300° C. and then quenching the bar; extruding a billet from the bar after heating to between about 500° C. and about 900° C.; and cold rolling the billet into a tube in at least two steps, with heat treatments between them at between about 550° C. and about 650° C.
 9. A method according to claim 1, wherein said alloy contains between about 800 ppm and about 1700 ppm O.
 10. A method according claim 1, wherein said alloy contains between about 50 ppm and about 600 ppm Fe.
 11. A method according claim 1, wherein said alloy in addition to Zr contains about 0.8 weight percent to about 1.2 weight percent Nb, about 50 ppm to about 600 ppm Fe, about 800 ppm to about 1700 ppm O, less than about 250 ppm C, less than about 150 ppm Si, less than about 1000 ppm S and in addition to that only impurities of a content which does not exceed that which is normally accepted in Zr or Zr alloys for applications in nuclear reactors.
 12. A cladding tube for nuclear fuel for a nuclear pressure water reactor, comprising a generally cylindrical tube component of a Zr-based alloy, wherein the alloying element, except for Zr, having the highest content in the alloy is Nb, wherein the Nb content in weight percent is between about 0.8 and about 1.2 and wherein no alloying element, except for Zr and Nb, in said alloy, has a content which exceeds about 0.2 weight percent, wherein said tube component has been finally annealed such that it has a structure that is partly recrystallized but not completely recrystallized and wherein the degree of recrystallization in said tube component is higher than about 40% and lower than about 95%.
 13. A cladding tube according to claim 12, wherein the degree of recrystallization in said tube component is higher than about 60% and lower than about 90%.
 14. A cladding tube according to claim 12, wherein said alloy contains between about 800 ppm and about 1700 ppm O.
 15. A cladding tube according to claim 12, wherein said alloy contains between about 50 ppm and about 600 ppm Fe.
 16. A cladding tube according to claim 12, wherein said alloy in addition to Zr contains about 0.8 weight percent to about 1.2 weight percent Nb, about 50 ppm to about 600 ppm Fe, about 800 ppm to about 1700 ppm O, less than about 250 ppm C, less than about 150 ppm Si, less than about 1000 ppm S and in addition to that only impurities of a content which does not exceed that which is normally accepted in Zr or Zr alloys for applications in nuclear reactors.
 17. A fuel assembly for a nuclear pressure water reactor, comprising: a plurality of cladding tubes, each having a generally cylindrical tube component of a Zr-based alloy, wherein the alloying element, except for Zr, having the highest content in the alloy is Nb, wherein the Nb content in weight percent is between about 0.8 and about 1.2 and wherein no alloying element, except for Zr and Nb, in said alloy, has a content which exceeds about 0.2 weight percent, wherein said tube component has been finally annealed such that it has a structure that is partly recrystallized but not completely recrystallized and wherein the degree of recrystallization in said tube component is higher than about 40% and lower than about 95%, and wherein each of said cladding tubes is filled with nuclear fuel suitable for such cladding tubes for a nuclear pressure water reactor; wherein the fuel assembly also comprises: a top plate, a bottom plate, a plurality of guide tubes for control rods, which guide tubes extend between the top plate and the bottom plate, and a plurality of spacers arranged for maintaining said cladding tubes in position in the fuel assembly and at suitable distances from each other.
 18. A fuel assembly according to claim 17, wherein the degree of recrystallization in said tube component is higher than about 60% and lower than about 90%.
 19. A fuel assembly according to claim 17, wherein said alloy contains between about 800 ppm and about 1700 ppm O.
 20. A fuel assembly according to claim 17, wherein said alloy contains between about 50 ppm and about 600 ppm Fe.
 21. A fuel assembly according to claim 17, wherein said alloy in addition to Zr contains about 0.8 weight percent to about 1.2 weight percent Nb, about 50 ppm to about 600 ppm Fe, about 800 ppm to about 1700 ppm O, less than about 250 ppm C, less than about 150 ppm Si, less than about 1000 ppm S and in addition to that only impurities of a content which does not exceed that which is normally accepted in Zr or Zr alloys for applications in nuclear reactors. 